1. Field of the Invention
This invention relates to a process for producing chlorine dioxide in which the process is superior both in terms of the equipment used and the economy achieved.
2. Description of the Prior Art
Canadian Pat. No. 782,574 discloses an electrolytic process for the production of chlorine dioxide starting with an aqueous solution of a metal chloride. In view of the fact that chlorine dioxide gas in concentrated form is spontaneously explosive, specific procedures and equipment must be employed in this process in order to minimize hazards as a result thereof. Thus, the equipment and process involved in conducting the process described in Canadian Pat. No. 782,574 is quite complicated.
Chlorine dioxide is often produced by reducing a chlorate, generally sodium chlorate. Known methods for producing chlorine dioxide involve reducing a chlorate with a strong acid such as sulfuric acid or hydrochloric acid in the presence of an alkali metal chloride.
Canadian Pat. No. 461,586 discloses a process for producing chlorine dioxide from hydrochloric acid in which the chlorine dioxide generator comprises a plurality of reaction vessels arranged in series. A liquid reaction mixture containing sodium chlorate, hydrochloric acid and sodium chloride is fed into the upper of a plurality of reactors from which the solution cascades by gravity through the remaining reactors. This process for the production of chlorine dioxide is not preferred since a plurality of reactors is involved and thus the equipment design and reaction mixture flow-through are complicated. The most efficient reduction method involves using hydrochloric acid as a reducing agent for the chlorate and a single reactor which also functions as a generator, an evaporator and a crystallizer, as disclosed in Canadian Pat. No. 969,735 (corresponding to British Pat. No. 1,347,740 and Japanese patent application (OPI) No. 15391/72) and U.S. Pat. No. 3,929,974 (corresponding to Japanese patent application (OPI) No. 59095/73).
Reduction of sodium chlorate with hydrochloric acid involves the following two reactions (1) and (2) EQU NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +NaCl+H.sub.2 O (1) EQU NaClO.sub.3 +6HCl.fwdarw.NaCl+3Cl.sub.2 +3H.sub.2 O (2)
To obtain chlorine dioxide with good efficiency, it is desirable to select reaction conditions which will promote reaction (1) while inhibiting reaction (2), for example by using a suitable catalyst or by selecting the proportions of the starting materials fed into the reactor such that reaction (1) will proceed mainly.
Sometimes, as described above to utilize sodium chloride in the spent reaction solution effectively, the recovered sodium chloride is fed as a starting material to an electrolytic cell for production of sodium chlorate.
For example, Canadian Pat. No. 825,084 discloses a process for the production of chlorine dioxide and chlorine and the production of an alkali metal salt in a single vessel by utilizing as starting materials an alkali metal chlorate, an alkali metal chloride and a strong acid in suitable proportions to generate chlorine dioxide and chlorine. However, after removal of the chlorine dioxide and chlorine generated in the reaction together with water vapor, the reaction mixture in the reaction vessel must be maintained at a sufficiently high temperature that water is removed from the reaction medium in order to crystallize out the alkali metal salt of the strong acid employed which can then be withdrawn and recovered from the reaction vessel as the alkali metal salt. Since an alkali metal chloride is often utilized as the material for production by electrolysis of the starting alkali metal chlorate, in the process of Canadian Pat. No. 825,084 only if hydrochloric acid is employed as the strong acid can recycling of the alkali metal chloride to an electrolytic cell for the production of alkali metal chlorate be considered and further without removal from the alkali metal salt recovered from the reaction system, after evaporation of water therefrom, of residual chlorine dioxide and chlorine gas can the alkali metal chloride be recycled to an electrolytic cell for the production of chlorine dioxide without explosion hazards arising.
Further, Canadian Pat. No. 826,577 discloses a process similar to that of Canadian Pat. No. 825,084, with the exception that sulfuric acid is employed as the strong acid and sodium sulfate is formed as a reaction product. Thus, the alkali metal salt formed, sodium sulfate, cannot be recycled to an electrolytic cell for the production of sodium chlorate for utilization as a starting material in the production of chlorine dioxide and chlorine.
The method disclosed in Canadian Pat. No. 969,735 comprises reacting hydrochloric acid with sodium chlorate in a reactor to form chlorine dioxide and chlorine, reducing the pressure of the reactor at that time, evaporating water at a temperature sufficient to crystallize sodium chloride, withdrawing a gaseous mixture of steam, chlorine and chlorine dioxide from the reactor, simultaneously recovering the solid sodium chloride precipitated, electrolyzing the recovered sodium chloride as an aqueous solution, and feeding the resulting sodium chlorate to the reactor mentioned above.
When the above method is operated under conditions which will precipitate sodium chloride, the slurry-like residual reaction mixture containing precipitated sodium chloride is withdrawn from the bottom of the reactor. The reaction mixture is separated into a solution and the solid sodium chloride. The solution is recycled to the reaction, and in the meantime, the solid sodium chloride is washed with water and re-dissolved, and fed into an electrolytic cell for the production of sodium chlorate. The solution of sodium chloride is electrolyzed to form sodium chlorate, and the sodium chlorate is fed into the reactor.
If the slurry-like residual reaction mixture containing the precipitated sodium chloride is fed continuously to the electrolytic cell for the production of sodium chlorate without the separating treatment, the unreacted hydrochloric acid remains in the residual reaction mixture and the pH of the residual reaction mixture becomes about 3 to 4 or lower. This tends to cause the generation of chlorine gas during the electrolysis in the electrolytic cell for production of sodium chlorate. Furthermore, since the chlorine dioxide and the chlorine present in the residual reaction mixture flow into the electrolytic cell, the chlorine can react with the hydrogen generated in the electrolytic cell giving rise to explosion hazards.
For this reason, this process described above requires a step for separating the residual reaction mixture and a step for washing and re-dissolving the sodium chloride, and therefore, the process steps become complicated. Moreover, according to this process, a reservoir for the residual reaction mixture is provided beneath the reactor to prevent a clogging of the withdrawal pipe because the withdrawal of the residual reaction mixture from the reactor is conducted intermittently, the reactor is at a reduced pressure, and the residual reaction mixture withdrawn contains a slurry of sodium chloride. Hence, the reactor must be installed at a high level above the ground, and the apparatus as a whole becomes large in size.
U.S. Pat. No. 3,929,974 discloses a process for producing chlorine dioxide by continuously feeding an aqueous solution of an alkali metal chlorate and hydrochloric acid into a reaction zone where chlorine dioxide and chlorine are continuously formed by the reaction between the alkali metal chlorate and the hydrochloric acid. During the reaction, the reaction medium is maintained at its boiling point to evaporate water from the reaction medium to form a gas phase in the reaction zone consisting of a mixture of chlorine dioxide, chlorine and water vapor which is then removed from the reaction zone. Two embodiments of the process are disclosed, one in which sodium chloride produced in the reaction is not precipitated but rather is removed as a liquid effluent continuously to maintain the liquid level in the reaction zone constant and another in which solid sodium chloride as a precipitate is removed from the reaction zone. In the first embodiment in which a liquid medium containing the sodium chloride is removed, such also contains dissolved therein chlorine dioxide and chlorine which is not stripped in the reaction vessel, and due to the presence of these materials a series of side reactions occur, some which are particularly dangerous when large quantities of hydrogen are mixed with the chlorine dioxide and chlorine and which reduce the efficiency of the electrolytic cell used to convert sodium chloride into sodium chlorate. In the second embodiment, in which solid sodium chloride is removed from the reaction zone, such is continuously dissolved in water and then fed to an electrolytic cell for the production of chlorate where it is converted into sodium chlorate and hydrogen. Unfortunately, just as was the situation with Canadian Pat. No. 969,735, this mixture containing solid sodium chloride removed from the reaction zone will also contain chlorine dioxide and chlorine which, when passed into an electrolytic cell, the chlorine can react with hydrogen generated in the electrolytic cell to give rise to explosion hazards.